The Periodic Table of Elements: the connected vocabulary

The terms in The Periodic Table of Elements are best learned as one connected system rather than a list to memorise, because almost every one of them flows from a single starting idea: an element’s position in the table. Once you see that the position fixes the electron arrangement, and the electron arrangement fixes the chemistry, the whole vocabulary turns from a set of separate definitions into one story that explains itself. That is also the fastest route to the precise wording the marking scheme rewards, since in this chapter the definition itself is often the mark.

The table and its coordinates. Everything begins with the periodic table: the arrangement of all the elements in order of increasing proton number, set out so that similar elements fall together. Two coordinates then locate any element. A group is a vertical column, and a period is a horizontal row. The group number tells you the number of valence electrons for a main-group element, while the period number tells you the number of occupied electron shells. Because a group shares its valence electrons, elements in the same group are chemically alike; because a period only shares its number of shells, elements in the same period are not. This is the first pair students confuse, group and period, and keeping them straight is worth many marks.

Trends across and down. Position also governs how properties change. The atomic radius shrinks across a period, because the nuclear charge rises while the number of shells stays fixed and pulls the same electrons in more tightly, and it grows down a group, because each element adds a whole new shell. Atomic size in turn drives reactivity: a larger atom loses its outer electron more easily and gains one less easily. This single idea explains two opposite trends at once, and being able to reason from atomic size to reactivity is one of the most heavily tested skills in the chapter.

The families. The group idea comes alive in three families. The alkali metals of Group 1 each have one valence electron, are soft and reactive, and form a metal hydroxide with water; they become more reactive down the group. The halogens of Group 17 each have seven valence electrons, exist as diatomic molecules, and form salts called halides; they become less reactive down the group, the exact reverse of Group 1. The noble gases of Group 18 have a full outer shell, an octet (or a duplet for helium), which makes them inert and monatomic. The halogens also give the clearest displacement reaction, in which a more reactive halogen sets a less reactive one free from its salt. Here students confuse several pairs: halogen with halide, alkali metal with alkaline earth metal, and octet with duplet.

The metalloids and the transition elements. Not every element is cleanly a metal or a non-metal; a few, such as silicon, are metalloids that sit along the dividing staircase with mixed properties. The transition elements of the central block are metals with four special properties: a variable oxidation number (iron as Fe²⁺ and Fe³⁺), catalytic activity, coloured compounds and complex ions. Period 3 also introduces the amphoteric oxide, such as aluminium oxide, which reacts with both acids and alkalis and marks the changeover from metal to non-metal across the row.

How the table was built. The chapter also carries a short history: the law of octaves of Newlands and the triads of Döbereiner were early hints of periodicity, before Mendeleev arranged the elements and the modern table settled on proton number as the ordering rule. Knowing which scientist did what is a common question.

These terms build directly on the atom you met earlier and lead straight into chemical bonding, where atoms react to reach the same stable octet the noble gases already have. Our teachers work through the vocabulary in this connected order in online one-to-one lessons, so that each definition reinforces the last and the precise wording that SPM Chemistry rewards becomes second nature under exam pressure.